Various devices have been described in the literature and patents which seek to accomplish the above objectives based on electronic devices which do not employ any direct mechanical connection between the driver's control device, whether a steering wheel or a “joy stick” or the like.
There is concern, however, at the absolute reliability of such devices and has been proposed that one or even two additional electronic systems be used to provide system redundancy to lessen the possibility of a major malfunction occurring. Alternatively, it has been proposed that a mechanical connection also be provided between the driver's control device and the road wheels which would take over in the event of a major malfunction of the electronic device.
A problem arises, however, in having a mechanical back-up device reasonably mimic the performance of the electronic device, so that, if a transition to mechanical from electronic operation occurred at high speed or in a corner, the driver has the best chance of retaining control of the vehicle.
Consideration must also be given to the appropriate ratio pattern relating the angle of turn of the road wheels to the steering wheel. Only certain mechanical linkages are suitable to perform this function, one of which is the well-known Geneva Pin and Slot device the use of which is described below. An alternative design using a cam mechanism is also described.
Practical means of providing the correct force levels (that is “feel”) at the steering wheel rim are required in both the mechanical and electronic devices.
It is now accepted that power assistance is provided in the steering systems of all but the smallest cars, and that electric power assist will replace hydraulic assist in many cases.
JP Patent publication no. 60131366A in the name of Fuji Heavy Industries Limited discloses a variable ratio steering gear in which the ratio between the driver's control device and the road wheels varies firstly in accordance with vehicle speed and secondly in accordance with the angle of turn from centre of the road wheels. However, the steering gear described in this reference is only for a manual steering gear in which the steering gear ratio is reduced over the central region of the rack by utilising a linkage means which creates movement of the pinion and housing simultaneously. The linkage means utilised results in further reduction of the steering gear ratio over the central region of the rack as vehicle speed increases. Such a device is not suited for use on a power assisted steering gear and is an undesirable arrangement in that the signal to increase the output near centre is derived from the modified output and not the input.
The following description relates to the appropriate mechanical construction which is suited to both hydraulic and electric power steering, both of which are based, according to the invention, on substantially the same construction.
Provision is also made within these mechanisms to provide supplemental steering inputs which may be derived from electronic devices, but according to the preferred embodiment such inputs will be very limited in extent so that even if they malfunction the effect on the driver's control will be minimal.